Maryland has a pretty amazing geologic history, with many eras represented within a relatively small area. I live within a zone called Upper Pelitic Schist, which is described as "Albite-chlorite- muscovite-quartz schist with sporadic thin beds of laminated micaceous quartzite."
Geologic Maps of Maryland: Montgomery County
The colors are highly variable and these veins of quartzite often have a very thin layer of silver or gold mica bonded to the surface, making the stones quite lovely in my view. The laminated characteristic would seem to make this ideal scraper material. Instead of the chunky irregularity of coarsely crystalline quartz, the laminated quartzite lays down in thin layers that exhibit very hard and sharp edges along easily worked planes. And there's often a curving undulation to the pieces, making them (ahem cough cough) comfortable in the hand.
Here's a few. BTW, I am not saying these are scrapers, just giving you a good view of the lamination.
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